The present invention relates to a stencil printer for printing an image on a paper or similar recording medium via a master wrapped therearound.
There has been known a stencil printer of the type perforating a stencil with, e.g., a thermal head in accordance with image data, wrapping the perforated stencil or master around an ink drum and pressing a paper fed at a preselected timing against the ink drum with a press roller or similar pressing member, thereby printing an image on the paper. In this type of stencil printer, the ink drum includes a hollow shaft supported by opposite side walls of the printer body. A pair of flanges are rotatably mounted on the shaft. A hollow cylindrical porous support is supported by the flanges at opposite ends thereof. An ink roller and a doctor roller or similar ink feeding means are arranged in the ink drum.
When the press roller presses a paper against the outer periphery of the ink drum, the support deforms and contacts the ink roller at its inner periphery. As a result, ink sequentially penetrates into the pores of the support, mesh screens, and perforations formed in a master. The ink oozing out from the master is transferred to the paper, forming an image on the paper.
However, the problem with the above conventional stencil printer is that the inner periphery of the ink drum cannot be evenly pressed against the ink roller in the axial direction of the drum. Specifically, because the support is rigid, its opposite end portions supported by the flanges deform little, compared to the intermediate portion. As a result, the amount of ink fed and therefore image density is not uniform. This is particularly true when temperature is low, e.g., in winter.
In light of the above, the rigidity of the ink drum may be reduced to allow the porous support to deform easily, as proposed in the past. This, however, brings about another problem that the area over which the ink drum and press roller contact each other increases in proportion to the easiness of deformation, i.e., the ink drum turns with respect to the press roller by a greater angle, resulting in excessive ink transfer. More specifically, when the amount of ink transfer increases, the viscosity of the ink increases and causes the paper to adhere more strongly to the ink drum. As a result, the paper is apt to roll up by being entrained by the ink drum in rotation without being peeled off at a preselected position.
Japanese Patent Laid-Open Publication No. 1-204781 discloses a stencil printer capable of maintaining the shaft of an ink drum and that of a press roller at a preselected distance and causing an displaceable ink roller to press the drum against a press roller at the inside of the drum. In the printer taught in this document, a hollow cylindrical portion included in the ink drum is formed only of an elastic material in order to allow the drum to deform outward in a convex configuration. This allows the inner periphery of the ink drum to contact the ink roller under uniform pressure in the axial direction. However, the contact area over which the ink roller and the inner periphery of the ink drum pressed by the ink roller increases, again resulting in excessive ink transfer and therefor the roll-up of a paper.
The prerequisite with the stencil printer of the type described is therefore that irregular image density be obviated without reducing the rigidity of the ink drum. Japanese Patent Laid-Open Publication No. 7-276773 teaches a stencil printer having a specific configuration for meeting the above requirement. In the specific configuration, a rigid ink drum includes a rotary shaft portion greater in diameter than a support shaft or ink feed pipe, so that the ink drum is movable up and down. A press roller is positioned beneath the ink drum. While the ink drum is not pressed by the press roller, the upper end of the rotary shaft portion rests on the support shaft due to the weight of the ink drum. In this condition, a gap formed between the support shaft and the lower end of the rotary shaft portion is greater than a gap between the ink roller and the inner periphery of the ink drum.
When the above press roller is moved to press the ink drum, the drum is raised until its inner periphery contacts the stationary ink roller. Because the ink drum is rigid, the press roller and ink roller contact the drum in a condition adequate enough to obviate excessive ink transfer. Moreover, because the hollow cylindrical portion of the ink drum does not deform, contact pressure acts evenly between the ink drum and the inner periphery of the ink drum in the axial direction and obviates irregular image density.
Even the stencil printer taught in the above Laid-Open Publication No. 7-276773 has some problems left unsolved, as follows. The press roller must exert a force great enough not only to raise the ink drum but also to implement a printing pressure. This increases the size of a drive source for driving the press roller and aggravates power consumption. Further, the gap between the rotary shaft portion of the ink drum and the support shaft is likely to bring about shaking due to vibration during, e.g., delivery, damaging the structural parts of the printer.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 6-135111 and Japanese Patent Application No. 10-88806.